How to make the player move without user input

If you want your player to move without user input like in Subway Surfers or other runners you can try using Tranlate https://docs.unity3d.com/ScriptReference/Transform.Translate.html Here is an example

 public float speed = 10f;
 private Vector3 dir;
 
 void Start(){
      dir = Vector3.forward; //Vector3(0, 0, 1)
 }
 
 void Update(){
      float amountToMove = speed * Time.deltaTime;
      transform.Translate (dir * amountToMove);
 }

For more information about Transform.Translate and others check Unity Scripting Reference https://docs.unity3d.com/ScriptReference/Transform.Translate.html